Device and method for fingerprints supervision

ABSTRACT

A method and a device for checking fingerprints are described. The method comprises the steps of recording in succession at least two digital images of finger areas and comparing each of the recording images with a reference image that represents at least one previously recorded reference fingerprint from a reference finger areas. The device according to the invention is arranged to carry out the method.

TECHNICAL FIELD

[0001] The present invention relates to a device that is intended to beused in a system for checking fingerprints, which device comprises asensor which is arranged to record a fingerprint from the first joint ofa finger. The invention further relates to a method for checkingfingerprints.

TECHNICAL BACKGROUND

[0002] It has been known from time immemorial that fingerprints can beused to identify people. Initially fingerprints were taken manually byinking the first joint of a finger and pressing this onto a sheet ofpaper. A pattern of lines was thus printed onto the sheet of paper,where the lines corresponded to the ridges and the spaces between thelines corresponded to the grooves in the skin of the finger. Nowfingerprints are taken using sensors that detect the pattern on the skinon the part of the finger that is held against the sensor.

[0003] Traditionally, fingerprint technology has been used primarily forthe identification of criminals, but in recent years it has also begunto be used for access control.

[0004] Known access control systems are based on a person who is to begiven access to something, recording his fingerprint under secureconditions. The recorded fingerprint is stored in a memory, for examplea database or on a personal data carrier.

[0005] When the access control is to be carried out, the person placeshis finger on a sensor which records a current fingerprint. Thisrecorded current fingerprint is compared with the previously recordedfingerprint or fingerprints that are in the memory, in order to checkwhether the recorded current fingerprint is the same as this previouslyrecorded fingerprint or any one of these previously recordedfingerprints and accordingly belongs to an approved person. If so, thesystem sends a signal indicating that such is the case. The signal cancontrol the access to, for example, premises, a computer or information,as applicable.

[0006] Checking whether a current recorded fingerprint is the same as apreviously recorded fingerprint is carried out using hardware orsoftware and on the basis of digital representations of thefingerprints. The fingerprint must be represented at high resolution, asonly small details distinguish different fingerprints from each other.This means that there is a large amount of information that must bestored for each fingerprint, and also that there is a lot of informationthat has to be compared in order to determine whether two fingerprintsoriginate from the same person.

[0007] One way of reducing the amount of information that is stored andcompared is to utilise “feature extraction”. According to this, thefingerprint is recorded from the person who is to be granted access towhatever is protected by the system, special features in the fingerprintare sought, such as points where the ridges divide and points where theridges end, and the positions of these features are determined. Only theposition information and the categorisation of the features is stored inthe system's memory. When a person's authorisation is to be checked,this person's fingerprint is recorded again and then a check is madewhether the same features recur in the same places in the currentrecorded fingerprint as in the previously stored fingerprint.

[0008] The sensors that are used in fingerprint systems are based onvarious detection techniques. There are optical sensors, whichessentially comprise a transparent surface, against which the user holdshis finger, a lens system and the sensor proper which has alight-sensitive surface. The lens system projects a reduced image of thefingerprint onto the sensor. Optical sensors have the disadvantage ofbeing unwieldy in design, on account of the lens system.

[0009] There are also so-called silicon sensors, where the user's fingeris in direct contact with the active surface of the sensor. At presentthe most common silicon sensors are capacitive. They have a sensorsurface of at least 100 mm². The capacitive silicon sensors arerelatively expensive as they require a large silicon area.

[0010] In addition, Atmel provides a thermal sensor with the nameFingerChip™. This sensor measures 1.5 mm×14 mm. When a fingerprint is tobe read, the user passes his finger over the sensor which records, at ahigh frequency and using thermal detection, “images” of the part of thefinger that is over the sensor at that moment. The “images” arethereafter put together into a single “image” of the whole fingerprint.The whole image is thereafter compared in the normal way with apreviously recorded image. This sensor requires a smaller silicon areathan the sensors that record a whole fingerprint from a finger that isheld still, but on the other hand is more difficult to use, as the usermust pass his finger over the sensor at a relatively even speed in orderfor the image to be clear. If the user passes his finger over the sensorslowly or even stops, the result will be impaired as the sensor is basedon temperature differences that are evened out immediately if the fingeris held still.

[0011] PCT application SE00/00268 describes a device which enables asmall sensor to be used. Instead of putting together several partialimages to form one large image, the small image that is obtained fromthe small sensor is compared with various parts of a large image. Byusing the complete image of the fingerprint instead of only features inthe image, good reliability is obtained without the sensor needing to belarge.

[0012] In certain cases, however, it can be desirable not to need tostore a whole fingerprint, due to the large amount of memory that isrequired.

[0013] There is thus a need for an alternative device for checkingfingerprints, which device is relatively simple and cheap.

SUMMARY OF THE INVENTION

[0014] An object of the present invention is to provide a device that isintended to be used in a system for checking fingerprints, which deviceis economical to manufacture.

[0015] A further object of the present invention is to provide a methodthat makes possible a simpler and cheaper device for checkingfingerprints.

[0016] These objects are achieved by means of a device and a methodaccording to the independent claims.

[0017] Further characteristics of the invention are apparent from theappended claims.

[0018] The inventors have discovered that it is possible to achieve goodsecurity while at the same time using a small sensor and a small memoryby comparing in succession small recorded images with a referencefingerprint.

[0019] A method for checking fingerprints according to the invention ischaracterised in that at least two digital images of finger areas arerecorded in succession. Each of the recorded images is compared with areference image that represents at least one previously recordedreference fingerprint from a reference finger area that is considerablylarger than each of the finger areas.

[0020] By storing the reference fingerprint in the form of a referenceimage that represents a reference fingerprint, only a small amount ofmemory is needed for the storage of the reference fingerprint if therecorded image is converted into an image without grey scale, that is animage which is binary. By comparing in succession recorded images withthe reference image, only the latest recorded image needs to be storedin a temporary memory. This means that it is possible to implement thecomparison in hardware that can be manufactured relatively cheaply,while at the same time the security of the checking is relatively good,as several recorded images are compared with the reference image.

[0021] The comparison between the recorded images and the referenceimage can advantageously be carried out between comparison images, whichare generated based on the recorded digital images, and the referenceimage, each comparison image representing at least a part of a recordedimage.

[0022] As the comparison images consist of at least a part of therecorded images, they take up less memory than the recorded images.

[0023] According to one embodiment, the comparison images consist of therecorded images.

[0024] It is preferable that at least one base area is provided, thecomparison image being compared with comparison areas in the referenceimage in the vicinity of said at least one base area. Thus, the firstgenerated comparison image is compared with the reference image in orderto find at least one base area that has at least a predeterminedconformity with the comparison area. The next generated comparison imageis thereafter compared with comparison areas in the vicinity of the basearea in order to investigate whether the comparison image matches partof the reference image. If no comparison area is found in the vicinityof a base area that has sufficiently good conformity with the comparisonimage, this base area is rejected without any new base area beingselected. There is thus a chain of comparisons, based on each of theoriginal base areas. After a small number of steps, only one base arearemains. The chain of base areas that preceded the remaining base areain the reference image corresponds to the path along which the fingerhas been moved during the recording of the images.

[0025] A preferred method according to the invention comprises the stepsof (i) providing at least one base area in the reference image, (ii)recording an image of a finger area, (iii) converting the recorded imageinto a comparison image which represents a part of the recorded image.The preferred method according to the invention also comprises the stepsof (iv) comparing the comparison image with at least one comparison areain the reference image in the vicinity of each of said at least one basearea and (v) selecting one of said comparison areas as base area if thecomparison image and the comparison area have at least a predetermineddegree of conformity. The steps (ii) to (v) are repeated until an endcondition has been fulfilled.

[0026] The end condition is preferably that images are recorded until acombined conformity is achieved between each of the base areas and thecorresponding comparison image. According to one method with this endcondition, each comparison obtains a comparison value. When the totalcomparison value for a plurality of comparisons exceeds a predeterminedcomparison value, then the recorded fingerprint is considered to conformwith the reference fingerprint.

[0027] The end condition can alternatively be that a predeterminednumber of comparison images have been compared with the reference image.

[0028] A device for checking fingerprints according to the inventioncomprises a sensor which is arranged to record digital images of fingerareas from the first joint of a finger. The device is characterised inthat it also comprises a comparing means which is arranged to generatein succession comparison images from recorded images, which comparisonimages represent features in each of the recorded images. The device isfurther arranged to compare in succession each of the comparison imageswith a reference image which represents features in at least onepreviously recorded reference fingerprint which represents a referencefinger area which is considerably larger than each of the finger areas.

[0029] A device according to the invention makes possible a smallmemory, as the recorded images are relatively small. Unlike previouslyknown methods for checking fingerprints, the whole of the recordedfingerprint does not need to be stored at once in the device, as therecorded images are not put together but are compared in succession andas each recorded image is small.

[0030] The comparing means is advantageously implemented in hardware asthis is the most rapid solution. As only a small amount of memory isrequired in a device according to the invention, a fast memory can beimplemented in hardware at a low cost.

[0031] Since more than one recorded image is compared with the referenceimage, the reference image can be a simplified image of the referencefingerprint. This enables the memory requirement for the reference imageto be small.

[0032] The device according to the invention is preferably arranged toselect in succession base areas in the reference image by comparing thelatest generated comparison image with the reference image, thecomparison images being compared with comparison areas in the referenceimage in the vicinity of said at least one base area. By selecting baseareas in succession, a chain is obtained of base areas that have beencompared with comparison images. The comparison between the comparisonimages and the comparison areas can be carried out in a plurality ofknown ways.

[0033] It is preferable for the comparison image to be generated basedon only a part of the recorded image, which part is the part of therecorded image that contains the most features.

[0034] By using only a part of the recorded image, the comparison can becarried out more quickly, while at the same time sufficiently goodaccuracy can be achieved by selecting the optimal part of the recordedimage, in which part there is the maximum number of features.

[0035] The device is advantageously arranged to record images until acombined conformity is obtained between each of the base areas and thecorresponding comparison image. According to one embodiment, with thisend condition, each comparison results in a comparison value. When thetotal comparison value for a plurality of comparisons exceeds apredetermined point total, then the recorded fingerprint is consideredto conform with the reference fingerprint.

[0036] Alternatively, the device is arranged to record images until apredetermined number of images have been recorded and compared.

[0037] The device has advantageously a sensor that has a length in therange 5-25 mm, preferably 10-20 mm, and a width in the range 0.1-5 mm,preferably 0.3-3 mm.

[0038] The sensor can be of any type. For example, it can be optical orcapacitive.

[0039] Of course, the different characteristics of the invention thatrelate to the method can also be implemented in the device according tothe invention, and vice versa.

BRIEF DESCRIPTION OF THE DRAWINGS

[0040] The present invention will now be described with reference to theattached drawings, in which

[0041]FIG. 1 shows a device according to the invention for checkingfingerprints.

[0042]FIG. 2 shows a reference image which is used for checkingfingerprints according to the invention.

[0043]FIG. 3 shows schematically how recorded images of a fingerprintare compared with the reference image.

[0044]FIG. 4 shows how a comparison image is generated from a recordedimage.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0045]FIG. 1 shows a device 1 according to the invention for checkingfingerprints. The device comprises a long and narrow silicon sensor,which is 1 mm high and 10 mm wide, and a comparison device 3 forcomparing an image recorded by the silicon sensor with a reference imagewhich was stored in the device previously. The device has an output 4via which an output signal can be sent indicating whether thefingerprint conforms with the reference fingerprint. The device isdesigned as a free-standing unit which is connected to a computer 5which is made available to the user if the reference image conforms withthe fingerprint that is recorded by the silicon sensor.

[0046]FIG. 2 shows a reference image 6 of a fingerprint. The black lines7 correspond to grooves in the fingerprint. The reference image has beenrecorded by the same sensor 2 as is used for checking the fingerprints.

[0047]FIG. 3 shows schematically the reference image 6 from FIG. 2. Thefunction of the device will now be described with reference to FIG. 3.When a finger is passed over the sensor 2, images of the finger willstart to be recorded by the sensor 2. The first recorded image isconverted to a comparison image 8, 9 which is compared with variousparts of the reference image 6. The comparison between the images can becarried out in a plurality of known ways. The comparison image has atleast a predetermined degree of conformity with the reference image 6 ina first comparison area and a second comparison area designated 8 and 9respectively. The first comparison area 8 and the second comparison area9 will thereafter be used as a first base area and a second base arearespectively in the continued comparison. The second image that isrecorded by the sensor 2 is used to generate a second comparison imagewhich is compared with a plurality of comparison areas 10, 11 below thebase area 8. For the sake of clarity, only a third comparison area 10and a fourth comparison area 11 are shown in the Figure. In reality, thesecond comparison image is compared with a large number of comparisonareas in various positions vertically and horizontally, and rotatedthrough various angles in relation to the reference image. However, allthe comparison areas are below the base area 8, as this corresponds tothe expected direction of movement of the finger. The comparison resultsin the third comparison area being used as a new first base area. In thesame way, a new second base area 12 is achieved. A third generatedcomparison image does not, however, conform sufficiently well with anyone comparison area below the new second base area 12, for which reasonthe second base area 9 is rejected. Generated third and fourthcomparison images conform, however, sufficiently well with a fifthcomparison area 13 and a sixth comparison area 14. For each generationof a new base area, a comparison value is calculated indicating how wellthe comparison image conforms with the comparison area. When the totalcomparison value for comparisons in the same chain exceeds apredetermined value, the fingerprint that was recorded in succession bythe sensor 2 is considered to conform with the reference fingerprintthat is represented by the reference image 6. By comparing thecomparison image with comparison areas that are displaced sideways, anysideways movements of the finger across the sensor can be taken intoaccount.

[0048] The generation of a comparison image from a reference image willnow be described with reference to FIG. 4. FIG. 4 shows a fingerprint 15within which a recorded image 16 is shown. The recorded image 16contains 8×200 pixels. In order to make the comparison with a referencearea sufficiently quick, a comparison image is generated 8×50 pixels insize, by selecting the part of the recorded image that has the mostfeatures. At the same time as selecting said part, the image isconverted so that the grey scale is removed. The comparison time isthereby minimised, while retaining the security of the comparison.

[0049] The embodiments described above are only to be regarded asexamples.

[0050] A person skilled in the art will recognise that the aboveembodiments can be varied in a number of ways without departing from theconcept of the invention. For example, the recorded image can be useddirectly as the comparison image.

1. A method for checking fingerprints, characterised by the steps ofrecording in succession at least two digital images of finger areas andcomparing each of the recorded images with a reference image (6) thatrepresents at least one previously recorded reference fingerprint from areference finger area that is considerably larger than each of thefinger areas for the purpose of checking whether the fingerprint thatcorresponds to the recorded images conforms to the referencefingerprint.
 2. A method according to claim 1, in which the comparisonbetween the recorded digital images (16) and the reference image (6) iscarried out between comparison images (17) generated on the basis of therecorded digital images, and the reference image, each comparison image(17) representing at least a part of a recorded image (16).
 3. A methodaccording to claim 1 or 2, in which at least one base area (8, 9, 10,13, 14) is provided, in which the comparisons with the reference image(6) are carried out with comparison areas (10, 11) in the referenceimage (6) in the vicinity of said at least one base area (8, 9, 10, 13,14).
 4. A method according to claim 1, 2 or 3, which comprises the stepsof (i) providing at least one base area (8, 9, 10, 13, 14) in thereference image, (ii) recording an image of a finger area, (iii)generating a comparison image (17) based on the recorded image, (iv)comparing the comparison image (17) with at least one comparison area(10, 11) in the reference image (6) in the-vicinity of each of said atleast one base area (8, 9, 10, 13, 14), (v) selecting one of saidcomparison areas as base area if the comparison image (17) and thecomparison area (10, 11) have at least a predetermined degree ofconformity, and repeating the steps (ii) to (v) until an end conditionhas been fulfilled.
 5. A method according to claim 4, in which the endcondition is that a predetermined number of comparison images have beencompared with the reference image.
 6. A method according to claim 4, inwhich the end condition is that a combined conformity is achievedbetween each of the base areas and the corresponding comparison image.7. A method according to any one of claims 2-6, in which the comparisonimage (17) is generated from a part of a recorded image (16), which partis the part of the recorded image (16) that contains the most features.8. A device for checking fingerprints, comprising a sensor that isarranged to record digital images of finger areas from the first jointof a finger, characterised in that it also comprises a comparing meanswhich is arranged to compare in succession each of the recorded images(17) with a reference image (6) that represents at least one previouslyrecorded reference fingerprint from a reference finger area which isconsiderably larger than each of the finger areas.
 9. A device accordingto claim 8, which is arranged to generate comparison images (17) whichare based on at least a part of each of the recorded images (16) andwhich is arranged to compare the recorded images (16) with the referenceimage (6) by comparing the comparison images with the reference image.10. A device according to claim 9, which is arranged to select insuccession base areas (8, 9, 10, 13, 14) in the reference image (6) bycomparing the latest generated comparison image (17) with the referenceimage (6), the comparison images (17) being compared with comparisonareas (10, 11) in the reference image in the vicinity of said at leastone base area (8, 9, 10, 13, 14), a plurality of base areas, all ofwhich are based on a common base area, forming a chain of base areas.11. A device according to claim 9 or 10, which is arranged to recordimages until a predetermined number of images have been recorded andcompared.
 12. A device according to claim 10, which is arranged torecord images until a combined conformity is obtained between each ofthe base areas and the corresponding comparison image.
 13. A deviceaccording to claim 9, 10, 11 or 12, which is arranged to generate thecomparison image (17) based on a part of the recorded image (16), whichpart is the part of the recorded image (16) that contains the mostfeatures.
 14. A device according to any one of claims 8-13, which has asensor (2) that has a length in the range 5-25 mm, preferably 10-20 mm,and a width in the range 0.1-5 mm, preferably 0.3-3 mm.